The role of cell adhesion molecules and cytokines in regulating lymphocyte trafficking through the central nervous system (CNS), long held to be a site of "immunological privilege", is being investigated. These regulatory factors will be studied at the level of the lymphocyte interaction with the cerebromicrovascular endothelium using in vitro models specific for each of the three major stages involved in lymphocyte transendothelial migration. These stages are 1) lymphocyte adhesion to the luminal endothelial surface, 2) lymphocyte penetration through the endothelium, and 3) lymphocyte interactions in the perivascular space. Endothelial cell cultures derived from isolated microvessels will serve as the model system for studying lymphocyte adhesion to the luminal surface of the endothelium. These cultures, grown on membrane-filters will be suitable for studying lymphocyte penetration across the endothelium. The fresh microvessel isolates, with their outer surface of extracellular matrix and pericytes exposed, will provide the in vitro model system for studying lymphocyte interactions with the perivascular space. A syngeneic (mouse lymphocyte - mouse endothelium) model will be used to study the effect of viral encephalitis on lymphocyte - CNS endothelial interactions. Allogeneic (human lymphocyte - human endothelium) and xenogeneic (human lymphocyte - bovine endothelium) will be used to study the effects of mitogen-induced lymphocyte activation and exogenous cytokines on lymphocyte - endothelial cell interactions. Within each model system five independent variables and three dependent variables will be studied. The independent variables are 1) state of health of the lymphocyte donor, 2) site within the body from which lymphocytes are obtained, 3) state of lymphocyte activation, 4) the presence of various monoclonal antibodies (MAbs) to cell adhesion molecules, and 5) the presence of various cytokines. the dependent variables to be measured are 1) the number of lymphocytes bound, 2) the classification of the bound lymphocytes, and 3) the adhesion molecules expressed for the process of lymphocyte binding.